We're talking about two different things. Passing the code for validity and making the book look the way you want it on a range of devices. The first is straightforward. The second is an art :-)

I'm not convinced the second is even possible. There are a number of popular Android reader apps that phone and tablet users might use for that do not support style sheets all that well, overriding them entirely in many cases, so even simply formatted books can look considerably different from one app to the next. They may remain readable, but looking like what you want, not so much.

Many of these apps are designed for the reader to format your book to look the way the reader wants it to.